STATIONARY PONDS ON THE SLUMGULLION LANDSLIDE—AN EXAMPLE OF BASAL TOPOGRAPHIC CONTROL OF LANDSLIDE SURFACE MORPHOLOGY

Ponds on landslides are often viewed as morphological indicators of recent movement. On landslides where movement is short lived (i.e., a year or less in duration), the location of ponds is attributed the location of landslide surface structures such as grabens and scarps. Landslides that continually move for time periods longer than a year are unusual, and ponds on these landslides have rarely been studied in any systematic way. The Slumgullion landslide in the San Juan Mountains of southwestern Colorado has been moving for about the last 300 years and has multiple ponds on its surface. We have studied eight ponds during 30 trips to the landslide between July 1998 and July 2007. During each trip, we have taken notes on variability in pond locations and water levels, taken ground-based photos to document pond water with respect to moving landslide material and vegetation, conducted global positioning surveys of the elevations of water levels, and mapped pond sediments on the landslide surface. Additionally, we have used stereo aerial photos taken in October 1939, October 1940, and July 2000 to measure topographic profiles of the eight pond locations and examine topographic changes over a 60-year period of time.

Results from field and photo observations, mapping, and measurements indicate that all pond locations have remained spatially stationary (for at least the last 60 years) while landslide material moves through the locations as if on a conveyor belt. Water levels during the monitoring period were sensitive to changes in the local, spring-fed, stream network; as well as to periodic filling of pond locations by sediment from floods and debris flows. For pond locations to remain stationary, the locations must mimic depressions along the basal surface of the landslide. The existence of such depressions indicates that the topography of the basal surface is irregular. These results suggest that, for landslides that experience prolonged, sustained movement, 1) landslide surface morphology can be used as a guide to the morphology of the basal slip surface, and 2) models and stability analyses of these landslides should attempt to incorporate irregular basal surface topography.